In nuclear reactors intended for the generation of power, nuclear fuel elements are customarily of the rod type. These rods are grouped in assemblies in which a large number of rods, arranged in parallel and closely spaced, are held in a generally rectangular, square, or other polygonal configuration. In water-cooled reactors, the rods are usually held in position by spacer grids which are made of comparatively narrow, thin plates arranged in "egg crate" construction, with intersecting members positioned at right angles to each other. Springs on some of the members force fuel rods against dimples, which are usually comparatively rigid and arranged opposite to the springs within cells formed by the intersecting grid members.
The flow of cooling water is parallel to the rods, and, in order to secure efficient cooling, it is highly desirable that the flow be made turbulent or swirling. It is also desirable to have fluid circulation between different parts of an assembly, and between assemlbies, since different rods generate different amounts of heat. In order to produce the desired motion of cooling water, it has been usual to provide deflectors within the assembly. The most common way the deflectors have been provided is directly on the positioning grids as shown, for example, in U.S. Pat. No. 3,379,619 to H. N. Andrews et al. Special forms of deflectors have been devised in order to improve the efficiency of mixing, as shown by U.S. Pat. No. 4,039,379 to John F. Patterson and Kirk P. Galbraith. It has also been known to provide special "mixing vane grids," located between the positioning grids, as shown in U.S. Pat. No. 3,395,077 to L. S. Tong et al.
Some grid designs which depart from the general descriptions given above are shown in U.S. Pat. No. 3,281,327 to J. Webb et al and U.S. Pat. No. 3,813,288 to B. Leaver et al. The Webb et al. patent shows a spacer grid made up of inclined plates, through which the fuel rods pass. The inclination of the plates is such as to cause them to act as deflectors for the cooling fluid. The Leaver et al. patent shows a grid made up of double grid members which, by their own flexibility, provide the "spring" action to hold the fuel rods. There is no provision made for deflecting the cooling water out of its path parallel to the fuel rods.
One of the power losses sustained by a nuclear reactor is the energy involved in forcing the cooling water through the reactor core. It is, therefore, highly desirable that the design of the grids, both those for supporting the fuel rods and those for causing circulation of the cooling water, offer as little resistance to flow as possible. It is, therefore, an object of this invention to provide a grid which gives optimum circulation of cooling water about the fuel rods, while offering the minimum resistance to flow. It is also desirable for the entire grid to be made of a metal which offers minumum neutron absorption during reactor operation.